This invention relates to a mobile radio data communication system in which portable radio terminal units transmit a carrier wave modulated with various data on such aspects of business activities as production, inventory and sale, while a stationary radio unit demodulates the modulated carrier wave into the transmitted data to appropriately process the data.
A conventional system of this kind (for example the "Radio Terminal System" described by T. Miyamoto et al. in the NEC RESEARCH & DEVELOPMENT, No. 50, pp. 62-68, July 1978) consists of a plurality of portable radio terminal units, a stationary radio unit, a terminal control unit and a computer. Each portable radio terminal unit further comprises a keyboard having 14 data input and seven function keys, a memory for storing data, a display for displaying data and a transmitter and a receiver for transmitting and receiving data, respectively. The stationary radio unit on the other hand comprises a transmitter for transmitting response data and the like from the terminal control unit and a receiver for receiving data from the radio terminal units. In such a system, the operator will enter data with keys on his radio terminal unit and store the data in its memory. The stored data are displayed. Then, as he depresses the transmission key if the idle indicator lamp is on, the stored data are transmitted from the transmitter in the form of, for example, a frequency shift keyed (FSK) wave. The FSK wave is demodulated after it is received by the stationary radio unit, and supplied to the terminal control unit over a wired transmission path.
The terminal control unit first corrects errors, if any, in the received data signals, edits them into a computer-processible format and transfers them to the computer. If any error is found uncorrectible, a response message is sent to the pertinent radio terminal unit via the stationary radio unit, and at the radio terminal unit there is lit an "Error" lamp to demand that the operator retransmit the data. In such an instance, the erroneous data are not transferred to the computer.
The computer, after having processed the transferred data, sends back a response message to the radio terminal unit via the terminal control unit and the stationary radio unit. At the radio terminal unit, the returned acknowlege signal and response message are subjected to error correction, and if the error is found correctible, the "OK" lamp is lit, accompanied by the displaying of the response signal.
When the operator uses a radio terminal unit of such a system while moving around, he has to manipulate the keyboard of the radio terminal unit held in his hands. This is a very cumbersome procedure, and moreover the size of the terminal unit, determined by the keyboard features, tends to be large. Furthermore, the presence of many keys often invites pressing of wrong keys when the operator is moving around. The radio terminal has to be equipped with a coder for converting the contact signals of the keyboard into digital signals and a memory for storing data because a plurality of such signals are transmitted at a time. It also needs a means for illuminating each of the many keys so that the unit can be used even in the dark. These factors, too, contribute to enlarging the size of the radio terminal unit.